Priority is claimed to German Patent Application No. DE 102 40 249.3, filed Aug. 31, 2002, the entire disclosure of which is incorporated by reference herein.
The present invention relates to a plant for the drying and treatment of a pourable mineral material, such as, for example, coal. Mineral material, when being extracted and treated, occurs usually in the form of a mixture of fragmentary or granular material and finest-grained and pulverulent material. So that this mixture can be separated into predetermined fractions, for example fragments, coarse-grained, finest-grained and pulverulent material, first the fraction above a predetermined grain diameter is separated as fragmentary material by screening. So that the coarse-grained fraction can be further separated from the finest-grained and pulverulent fraction from passage through the screen, the screen passage must first be dried. Rotary drums have proved appropriate as an assembly for the drying of mineral mixtures.
Rotary drums for the drying of moist mineral material are typically drums which are inclined in the conveying direction of the material to be dried and through which hot smoke gases from a combustion plant are introduced either in countercurrent or in cocurrent to the material to be dried. Such plants are operated in cocurrent when the freshly fed moist material is to be brought into contact with the freshly introduced hot smoke gases in order to achieve rapid drying success, while the countercurrent method is employed, for example, when the already predried material is to be brought into contact with the hot smoke gas in the state of maximum drying capacity in order to expel the last moisture residues.
Plants of the type described are also referred to as rotary tubular kilns. A rotary tubular kiln operated in countercurrent is described in German Patent Document DE 42 43 264 A1. Further rotary kilns are described, for example, in European Patent Document EP 0 032 468 A, French Patent Documents FR 2 441 682 A and FR 1 116 508, U.S. Pat. No. 4,189,300, European Patent Document EP 0 030 403 A, Belgian Patent Document BE 858 730, and German Patent Documents DE 31 10 380 A1 and DE 38 15 104 A1.
A rotary tubular kiln, which is operated in cocurrent is known from U.S. Pat. No. 4,318,620.
Completely different aims in the treatment of the material during drying may be pursued with regard to the various types of mineral materials. If the mineral material is to be dried as far as possible without further abrasion and destruction, the material is treated carefully and protectively in the rotary drum. If, however, a mixture consists of hard dimensionally stable grains and of soft grains tending to disintegrate, stabilization can be achieved, during drying, in that the grains, while being dried and while traveling through the rotary drum, are deliberately subjected to harsh treatment, with the aim of destroying the softer grains, so that these occur as finest-grained or pulverulent material.
An object of the present invention is to provide a plant for the drying and treatment of pourable mineral material, in which stabilization intensity is high and good environmental compatibility is afforded.
The present invention provides a plant for the drying and treatment of pourable mineral material, that includes a rotary drum (1) inclined in the conveying direction of the material to be treated and provided with installed fittings (3, 4, 5) on the inner circumference, transport blades (3) being arranged as installed fittings in the front introduction region for the material to be treated, and the outlet end of the rotary drum (1) being connected sealingly to a shaft (6) which takes over the material emerging from the rotary drum (1) and the gas stream. The plant also includes a combustion apparatus (2), the smoke gases of which can be led through the rotary drum (1) in cocurrent with the material to be treated, a fan (7) which puts the shaft (6) under a vacuum, and a Venturi fitting (8) which is arranged coaxially in the rotary drum (1) upstream of the outlet end of the latter. A bypass (9) for conveying air is arranged in the drum casing in the introduction region of the material to be treated.
The plant according to the invention has the advantage that the fed moist mixture is brought into contact, immediately when being fed into the rotary drum, with the fresh hot smoke gases not yet cooled and is heated in a shock-like manner. As a result, the soft grains, which preferentially absorb water, disintegrate by decrepitation. The material to be dried, when it passes further on through the rotary drum, is lifted into a position in the upper region of the rotary drum by the installed fittings, so as then to fall downward in the drum over as long a distance as possible, further stabilization occurring. The smoke gas stream sucked in the conveying direction forms a strong flow, which picks up and carries along the pulverulent and finest-grained constituents. The flow velocity is increased by means of the Venturi fitting. The bypass for conveying air in the drum casing ensures that, in the event of a relatively high vacuum in the drum, additional conveying air is sucked in from the surroundings, without the air/fuel mixture in the combustion plant and consequently optimum combustion being disturbed by this additional conveying air, with the result that additional NOx formation is also very substantially avoided.
By a fan of variable rotational speed being used, it is advantageously possible to influence the suction and consequently the vacuum in the drum. The stabilization intensity can thereby also be influenced. Since the gas streams sucked through the Venturi fitting are dust-laden, the fact that the guide surfaces are formed only partially has the effect that no materials are accumulated or build up in the Venturi fitting, the result of this being that the flow resistance in the drum would increase.
If the inlet end of the drum is connected sealingly to the combustion apparatus, the intake of fresh air, which is not preheated is avoided. Since the bypass orifice in the drum casing is arranged only at a distance downstream of the inlet end, this has the effect that the ambient air sucked in directly at the inlet end is preheated when it flows through the annular gap between the drum casing and the double casing, so that the heating is utilized to assist the drying and the drying is not adversely influenced by the intake of cool ambient air which is not preheated. The thermal insulation of the double casing and of the circumferential boxes has the effect that the heat radiated out of the drum becomes active in the preheating of the bypass gas and is not lost due to radiation into the surroundings.
The design of the installed fittings alternately as straight plates and as plates bent in the longitudinal direction so as to form pockets has the effect of intermixing the material to be dried and of lifting the material to be dried into the uppermost regions of the drum volume during the rotation of the drum. The fact that a stop wheel rotatably mounted at a fixed location is arranged in such a way that at least one circumferential ring rolls against it advantageously prevents the rotary drum from being moved in the direction of inclination. The formation of the upwardly directed gas stream in the shaft leads to wind sifting of the discharged material in such a way that the granular material falls downward counter to the gas stream, while the finest-grained and pulverulent material, together with the gas stream, is led upward to the device for transfer into a dedusting plant.